A radiocolloid study was carried out, after splenectomy, in a 75-year-old man who had idiopathic thrombocytopenic purpura. The patient, who did not have an increase in platelet counts after spleen removal or after platelet infusions, showed radiocolloid accumulation in the lungs and in the bone marrow. This suggested some"activation" of reticuloendothelial cells, perhaps by circulating immune complexes. The left femur (site of an episode of poliomyelitis many years previously) had less radiocolloid uptake than the right.